Series connection of interdigitated surface wave transducers

ABSTRACT

In a transducer in which a plurality of acoustic sections are cascaded in series, a plurality of electrical sections are also cascaded in series to produce a relatively long interdigitated surface wave transducer having a relatively large input impedance. In one embodiment, electrical cascading is effected so that the interconnected acoustic sections are in phase with each other. In a different embodiment the series electrical cascading produces a phase reversal between the interconnected acoustic sections.

United States Patent Jones et al.

1451 July4, 1972 SERIES CONNECTION OF INTERDIGITATED SURFACE WAVETRANSDUCERS William Stanley Jones, Richardson; Clinton S. Hartmann,Dallas, both of Tex.

Inventors:

Primary Examiner]. D. Miller Assignee: Texas Instruments Incorporated,Dallas, Assistant Examiner Mark Budd Attorney-James 0. Dixon, Andrew M.Hassell, Melvin Shar p Filed; 2 1970 Harold Levine, Henry T. Olsen, GaryC. l-Ioneycutt, Michael A. Sileo, Jr., John E. Vandigriff and Richard L.Donaldson App]. No.: 94,305

{57] ABSTRACT US. Cl. ..310/9.8, 310/8.l, 333/30 R, In a transducer inwhich a plurality of acoustic sections are 333/72 cascaded in series, aplurality of electrical sections are also Int. Cl. ..I-I0lv 7/00cascaded in eries to produce a relatively long interdigitated Field ofSearch ..310/s, 8.l,9.7, 9.8; 333/30, Surface wave transducer having arelatively large input 333/72 pedance. In one embodiment, electricalcascading is effected so that the interconnected acoustic sections arein phase with References Cited each other. In a different embodiment theseries electrical cascading producesa phase reversal between theintercon- UNITED STATES PATENTS nected acoustic sections.

3,551,837 12/1970 Speiser et al ..3 l0/9.8 X 16 Claims, 13 DrawingFigures 36 1 1 h H744 44d I j 1 38b I 1 42 t 45; 38b l PATENTEUJUL 4 m23, s75 054 SHEET 2 OF 3 3 REX-b Fig. 3a

ELECTRIC FIELD STRENGTH Fig JIUMUJL Fig.5b

SERIES CONNECTION F INTERDIGITATED SURFACE WAVE TRANSDUCERS Thisinvention pertains generally to surface wave devices and moreparticularly to interdigitated surface wave transducers electricallycascaded in series.

The surface acoustic wave technology is ideally suited for applicationsin a wide range of passive and active signal processing systems delaylines, matched terminations, attenuators, phase shifters, bandpassfilters, pulse compression filters, matched filters, amplifiers,oscillators, mixers, and limiters, due to the ability to tap, guide,amplify and otherwise manipulate an acoustic wave as it propagates alongthe surface of a suitable substrate. Such devices utilize acoustic waveswhich propagate along a stress free plane surface of an isotropicelastic solid. These acoustic surface waves have an essentiallyexponential decay of amplitude into the solid and therefore most of theparticle displacement of the solid occurs within about one wavelength ofthe surface. For ease in coupling electrically to the surface waves,piezoelectric anisotropic substrates have generally been used. For suchpiezoelectric substrates coupling a signal to the surface wave can beaccomplished by means of deposited interdigitated metal electrodesspaced apart by one-half wavelength at the resonance frequency desired.

Usually, adjacent electrodes of an interdigitated transducer areelectrically connected in parallel and at relatively high frequencies,or when relatively long arrays are required for frequency selectivity orfor signal processing, the transducer exhibits an extremely low inputimpedance. For example, interdigitated transducers having 50 pairs ofelectrodes at 500 MHz may exhibit an input impedance in the range ofabout lohms on a lithium niobate substrate. Such a low impedance makesit extremely difficult to effectively utilize the transducer.

One technique that has been proposed to increase the electrical inputimpedance of an interdigitated surface wave transducer is to physicallyinterconnect several interdigitated sections in series so that theelectrical series impedance of the respective sections add. A majorproblem using external bonding wires, however, is the fact that thebonding wires used to interconnect the varioussections introduce phasedistortion which deleteriously affects the operation of the transducerand renders it inoperable for many applications.

Accordingly it is an object of the present invention to provide aninterdigitated surface wave transducer having a relatively high inputimpedance.

A further object of the present invention is to electrically andacoustically cascade in series a plurality of individual sections toform a relatively long interdigitated surface wave transducer having arelatively high electrical input impedance and uniform phasecharacteristics along the length thereof.

Another object of the present invention is to provide an electricallyand acoustically series cascaded interdigitated surface wave transducerthat is compatible with conventional transducer pattern metallizationtechniques.

Briefly and in accordance with the present invention, an interdigitatedsurface wave transducer having a relatively large input impedancecomprises a plurality of series cascaded acoustic sections and seriescascaded electrical sections, in which all electrodes may be formed in asingle metallization step. Successive electro-acoustic sections of thetransducer may be interconnected by a series in-phase connection oralternately successive electro-acoustic sections may be interconnectedby a series phase-reversal connection. Each electro-acoustic sectioncomprises a plurality of interdigitated electrodes that define apredetermined number of electric field interaction regions, theelectrical impedance of each section being relatively small with respectto thedesired transducer input impedance. Successive electro-acousticsections are interconnected by metal electrode coupling means thateffectively couple current from one electro-acoustic section to thesucceeding electro-acoustic section. When a series "inphaseinterconnection is required, adjacent interaction regions of successiveelectro-acoustic sections are spaced apart by three-fourths of anacoustic wavelength at the resonance frequency. When a seriesphase-reversal interconnection is desired, adjacent interaction regionsof successive electroacoustic sections are spaced apart by one-fourth ofan acoustic wavelength.

The novel features believed to be characteristic of this invention areset forth in the appended claims. The invention itself, however, as wellas other objects and advantages thereof may best be understood byreference to the following detailed description of illustrativeembodiments when read in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts an illustrative embodiment utilizing the series cascadedinterdigitated transducer of the present invention;

FIG. 2a depicts an embodiment of the present invention wherein threeacoustic sections are cascaded in phase;

FIG. 2b depicts the electric field produced by the interaction ofadjacent electrodes of FIG. 2a;

FIG. 3a depicts an alternate embodiment of successive acoustic sectionscascaded in phase;

FIG. 3b depicts the electrical field of the alternate embodiment shownin FIG. 3a;

FIG. 4:: depicts a modification of the embodiment shown in FIG. 3a;

FIG. 4b depicts the electric field of the modified embodimentillustrated in FIG. 40;

FIG. 5a depicts successive acoustic sections cascaded in series toproduce a phase reversal between sections;

FIG. 5b depicts the electrical field produced by the embodiment of FIG.5a;

FIG. 6 depicts an alternate embodiment for cascading successive acousticsections in series to produce a phase reversal;

FIG. 7 diagrammatically depicts a surface wave transducer that is phasecoded to represent a predetermined binary code;

FIG. 8 diagrammatically depicts a phase coded surface wave transducerthat is amplitude weighted so that the various acoustic sections of thetransducer exhibit the same effective impedance; and

FIG. 9 depicts in graphical form the theoretical and measured impedanceof a series cascaded transducer in accordance with the present inventionas compared to the input impedance of a corresponding conventionalinterdigitated transducer.

With reference now to the drawings and for the present particularly toFIG. 1, there is depicted partially schematically and partially in blockdiagram form an illustrative example incorporating a series cascadedinterdigitated transducer in accordance with the present invention. Thistransducer is shown generally at 10 and comprises seven discreteelectro-acoustic sections shown at 12a through 12g. As represented bythe arrows shown generally at 14, these individual electro-acousticsections are cascaded in series to form a relatively long interdigitatedtransducer having a relatively large input impedance. Theelectro-acoustic sections are disposed so that they form a singleacoustic channel, i.e., a propagating acoustic surface wave sequentiallytraverses successive acoustic sections. The use of seven acousticsections is by way of example only, and any desired number of acousticsections may be cascaded in accordance with the present invention.

Each of the electro-acoustic sections 12a through 12g comprises aninterdigitated array of electrodes and exhibits a relatively low inputimpedance with respect to the desired effective input impedance of thetransducer 10. An r.f. generator 16 applies an input signal across aninput terminal 18 of acoustic section 12a and across an input terminal20 of the acoustic section 123. An output transducer is shown in blockdiagram format at 22. Transducer 22 may, for example, comprise abroadband interdigitated surface wave transducer. Transducers l0 and 22are formed on a piezoelectric substrate 24 by conventional metallizationtechniques. In response to a signal from the r.f. generator 16, theinput transducer I0 generates an acoustic surface wave in the surface ofthe piezoelectric substrate 24. This surface wave is detected by theoutput transducer 22 and a signal is generated across the load 26.

i With reference now to FIGS. 2a and 2b, there is depicted anillustrative embodiment of the present invention wherein threeelectro-acoustic sections 28, 30 and 32 are cascaded by series in-phaseconnections. That is, successive electroacoustic sections such as 28 and30 are connected in series and are in phase with each other. Individualelectrodes of the interdigitated transducer are shown generally at 38aand 38b. These electrodes are preferably formed to be one-fourth of anacoustic wavelength at the desired resonance frequency in width.Adjacent electrodes 38a and 38b are spaced apart by one-fourth of awavelength, defining a center-tocenter spacing of adjacent electrodes ofone-half of a wavelength. Electrodes 40 and 42 that interconnectsuccessive electro-acoustic sections are respectively formed to bethree-fourths of a wavelength in width to maintain the correct phase incascading two discrete electroacoustic sections in series and to reducedistortion. This will be explained in more detail hereinafter in thedescription of FIGS. 3b and 4b. The electrodes 38a and 38b andelectrical pads or terminals 34 and 36 are formed by conventionalmetallization techniques on a piezoelectric substrate (not shown).

Responsive to an r.f. signal applied across terminals 34 and 36,adjacent electrodes 38a and 38b interact to produce an electric fielddisturbance in the surface of the piezoelectric substrate. As may beseen with reference to FIG. 2a, each electro-acoustic section, such assection 28, has five regions of electrode interaction shown at 44a, 44b,44c, 44d and 44e.

Referring now to FIG. 2b, there is depicted the electric field producedby the interaction between adjacent electrodes such as 38a and 38b ofFIG. 2a. Regions 48a and 48b depict the electric field produced by aconventional interdigitated transducer while the solid line 46 depictsthe electric field produced by the series cascaded interdigitaltransducer of the present invention. As may be seen, the wide electrodes40 and 42 that interconnect successive electro-acoustic sections shortout the substrate surface and prevent an electric field disturbance inareas where it would normally occur in conventional interdigitatedtransducers thus producing a slight distortion. However, it is to benoted that the phase of the electric field remains the same for each ofthe cascaded electroacoustic sections 28, 30 and 32, since the spacingtherebetween is three-fourths of a wavelength.

With reference to FIGS. 3a and 3b, there is depicted an alternateembodiment of the present invention for effecting inphase seriescascading of two acoustic sections. In this embodiment, a crossover orinterconnecting electrode 50 physically connects the twoelectro-acoustic sections 52 and 54 in series. To maintain the electricfield produced by section 54in phase with the electric field of section52, the adjacent interaction regions of acoustic sections 52 and 54,depicted at 49 and 53 respectively, are separated by three-fourths of awavelength. Or, stated another way, the separation or low interactionregion between the crossover electrode 50 and the adjacent electrode 51is formed to be three-fourths of a wavelength.

As may be seen with reference to FIG. 3b, the series inphase connectionbetween electro acoustic sections 52 and 54 introduces some distortionof the electric field due to the three-fourths of a wavelength region oflower interaction. The actual electrical field of the series cascadingarrangement shown in FIG. 3a is depicted by the solid line 56. Theelectric field associated with the low interaction region is shown at 55and the amplitude there is only about one-third of the amplitudeproduced in other interaction regions between adjacent electrodes. Thereason for this is that the lower interaction region 57 (FIG. 3a) isthree normal interaction regions in width, thereby producing acorrespondingly lower electrical field between the adjacent electrodes51 and 50. The dotted portion 58 depicts the electric field obtainedwith a conventional (i.e., non-cascaded) interdigitated transducer.

With reference to FIGS. 40 and 4b, there is depicted a modification ofthe series in-phase cascading arrangement of FIG. 3a. In this modifiedarrangement the crossover electrode 50' is formed so that it isthree-fourths of an acoustic wavelength in width. Forming the crossoverelectrode in this manner reduces the distortion produced in the electricfield as may be seen by comparing the response illustrated in FIG. 4b

with that depicted in FIG. 3b. With reference to FIG. 4b, the I dottedportion 58' represents the electric field that is obtained in aconventional interdigitated transducer while the solid line 56represents the actual electric field obtained with the arrangement ofFIG. 4a.

In-phase cascading of discrete electro-acoustic sections is preferablyaccomplished in accordance with the cascading arrangement depicted inFIG. 2a, since the resistive loss is less than that resulting fromeither of the cascading arrangements depicted in FIG. 3a or FIG. 4a.This may best be seen by reference to FIG. 2a wherein the current I thatflows between point A of acoustic section 30 and point B of acousticsection 32 is channeled through the coupling electrode 42. The distancebetween points A and B is two and one-half acoustic wavelengths. Theresistance between points A and B causes an unrecoverable IR voltagedrop that reduces the efficiency of the transducer.

With reference now to FIGS. 3a and 4a, the current con necting twosections 52 and 54 flows through the crossover electrode 50 from point Cto D. The distance between points C and D is significantly greater thanthe distance between points A and B (FIG. 2a) and thus the resistance ismuch greater, producing a much larger IR drop. For example, a typicaldistance from C to D may be on the order of wavelengths or greater.

With reference to FIG. 50, two sections 60 and 62 are cascaded in seriesso that a phase-reversal between the two sections is obtained. A phasereversal is obtained by forming a coupling electrode 61a betweenadjacent interacu'on regions of the two electro-acoustic sections, saidelectrode being onefourth a wavelength in width and being connected toan electrical pad 57 that is common to both acoustic sections. Thisphase reversal may be seen by reference generally to FIG. Sb, whichdepicts the electric field produced by interaction of adjacentelectrodes 61a and 61b, and specifically to portions 63a and 63b of theelectric field response.

An alternate arrangement for cascading acoustic sections in series toproduce phase reversal is depicted in FIG. 6. In this embodiment aninterconnecting electrode 64 physically connects electrical pad 65 ofone acoustic section with electrical pad 65' of the succeeding acousticsection.

In some applications it is desirable to increase the input electricalimpedance of phase-coded interdigitated arrays. One such application,for example, would be a low compression ratio phase-coded correlator ona higher coupling material or a very long phase-coded correlator havinga low compression ratio on a lower coupling material.

With reference to FIG. 7, a 13-bit Barker code of 1111100110101 isdepicted. By way of example, only, two electrode pairs are shown asrepresenting each binary bit. The binary state is changed, for example,from a l to a 0, by reversing the phase of the surface wave produced bythe transducer. This phase reversal may be effected most simply byconnecting two adjacent electrodes such as 66 and 67 to the sameelectrical pad 68. A binary coded array such as depicted in FIG. 7 maybe produced in accordance with the present invention utilizing, forexample, the in-phase series cascading depicted in FIG. 2a and thephase-reversal series cascading shown in FIG. 5a. In producing the phasecoded array it may be desirable to series cascade each electro-acousticsection corresponding to a discrete binary bit of the desired binarycode. In other words, assuming two electrode pairs are utilized for eachbinary code, such as is shown in FIG. 7, each group of two electrodepairs would constitute an electro-acoustic section that is to becascaded in series with succeeding electroacoustic sections required toformulate the desired 13 bits. That is, for a 13-bit binary code, 13acoustic sections would be cascaded in series. This arrangement may bedesirable even if adjacent acoustic segments are of the same polarity orphase, since otherwise the acoustic sections would have differentimpedance values leading to different acoustic contributions. Forexample, consider the amplitude binary code 1 1 10010. If this code wereconstructed as follows 11l*00*1*0 where the asterisks represent seriescascading in accordance with the present invention, the 111 acousticsection would have a lower impedance since three groups of electrodepairs are in parallel while the l and acoustic sections would-bothexhibit relatively high impedances. Thus, it may be desirable torepresent each binary bit with a separate acoustic segment and form thecoded array by interconnecting the acoustic sections as follows:1*l*l*0*0*1*0, wherein the asterisks denote series cascading. Wheninterconnecting like polarity sections, in-phase series cascading inaccordance with the present invention is utilized while phase-reversalseries cascading is utilized where the binary state of adjacent acousticsections is different.

If it is not desired to divide each binary bit into a separateelectro-acoustic section, an alternate cascading technique may beutilized wherein the total number of electrode pairs required torepresent thebinary code is determined. The electrode pairs may then beevenly divided into a convenient number of acoustic sections, therebyensuring that each acoustic section exhibits the same impedance. Thenecessary phase coding may then be effected to form the desired binarycode by cascading the acoustic sections (to efiect a relatively highinput impedance) in series in-phase or series phasereversal connections,as required. To obtain phase reversals within a discrete acousticsection, conventional phase reversing techniques may be employed, asshown at 66 and 67 in FIG. 7.

A different technique for ensuring that the effective impedance is thesame for different acoustic sections of a phase coded array is depictedin FIG. 8 wherein a portion of a coded array is shown and depicts abinary code of l 1 10010. By way of example, each binary bit isrepresented by two electrode pairs, one electrode pair being shown at70a and 70b. For increased clarity of description, the phase coded arrayis illustrated as being formed utilizing phase-reversal series cascadingin accordance with the embodiments depicted in FIG. a. It is to beappreciated, of course, that other techniques for series phase-reversalcascading may be utilized.

As illustrated in FIG. 8, the desired pattern of l 1 10010 may beobtained by cascading four electro-acoustic sections as follows111*00*1*O where the asterisk refers to series phase-reversal cascading.The interaction length between adjacent electrodes of each acousticsection is amplitude weighted a predetermined amount so that theeffective impedance of each acoustic section is the same. That is, theeffective impedance of an electrode pair is inversely proportional tothe interaction length. By weighting the acoustic section representingthree binary bits I 11 so that the interaction length of respectiveelectrode pairs is small, the parallel combination of the electrodepairs of that acoustic section is controlled to be relatively large.With respect to the electroacoustic section representing the individualbinary bits 1 and 0," the interaction length of electrode pairs thereinis relatively large, effecting an impedance that is the same as theparallel combination of the acoustic section representing the binarybits 1 l 1." Similarly, the interaction length of electrode pairs of theacoustic section representing 00 is controlled to an intermediate valueso that the effective impedance thereof is the same as the otheracoustic sections. For example, assuming that the interaction length ofelectrode pairs of the acoustic sections representing single binary bitsis one unit, the interaction length associated with acoustic sectionsrepresenting two binary bits and three binary bits may be controlled tobe three-sevenths and one-fourth units respectively (given by the numberof active electro-acoustic interactions per section). The requiredacoustic surface wave beam width W is controlled to be the same width asthe smallest interaction length so that the correct acoustic energy iscontained in that width since the acoustic amplitude has been devised tobe constant utilizing this method. This is done by using aninterdigitated transducer having an interaction length the same as thesmallest interaction length of the weighted coded array 73 as is shownat 72. Since interdigitated transducers are bi-directional, transducer72 may be utilized as an input transducer when the coded array 73 isfunctioning as a detector, or as an output transducer whenever the codedarray functions as a generator.

In accordance with the present invention, an interdigitated surface wavetransducer comprising three in-phase series cascaded acoustic sectionswas constructed. Each acoustic section was formed to have seveninteraction regions. The electrodes were formed to define a resonancefrequency of 21 MHz. Respective interaction regions were formed to havea length of approximately 90 mils, and respective electrodes within asection were formed to be one-fourth of an acoustic wavelength in' width(approximately 1 V1 mils). The electrodes were formed on a lithiumniobate substrate using conventional photomask and metallization etchingtechniques. The electrodes were formed of aluminum deposited on alithium niobate substrate to a thickness of about 2,000 A. The impedanceof the in-phase series cascaded interdigitated transducer abovedescribed was measured and the results are shown in graphical form at inFIG. 9. The measured impedance was compared with the theoreticalimpedance of a conventional interdigital surface wave transducer of thesame length. (Such a transducer has 24 interaction lengths.) Theimpedance was calculated in accordance with the equivalent circuit modeldescribed in Smith et al., Analysis of Interdigital Surface WaveTransducers by Use of an Equivalent Circuit Model," IEEE Transactions ofMicrowave Theory and Techniques, Vol. MTT-l7, No. 11, November, 1969.This calculated impedance isshown in FIG. 9 at 82. The theoretical valueof the series cascaded surface wave transducer was also calculated andis shown at 84. As may be seen, the theoretical impedance of aconventional long interdigital surface wave transducer is approximately860 ohms; the measured impedance of the series cascaded surface wavetransducer in accordance with the present invention is approximately8,600 ohms, while the theoretical impedance of the series cascadedtransducer is approximately 10,000 ohms. As may be seen, cascading threeacoustic sections increases the electrical impedance by a factor ofapproximately nine.

The impulse response of the series cascaded transducers above describedwas checked to determine whether any unwanted phase shifts were inducedby the series cascading; no unwanted phase shifting was observed.

Although specific embodiments of this invention have been describedherein, it will be apparent to a person skilled in the art that variousmodifications to the details of construction shown and described may bemade without departing from the scope of the invention.

We claim:

1. A relatively large input impedance interdigital surface wavetransducer that is phase coded to correspond to a predetermined binarycode, comprising in combination:

a. a piezoelectric substrate;

b. a plurality of spaced apart electro-acoustic sections disposed onsaid substrate to define a single acoustic channel, each of saidsections comprising two electrical pads having at least oneinterdigitated electrode pair selectively connected thereto and having arelatively small electrical impedance, the relative phase of each ofsaid electro-acoustic sections corresponding to a binary bit of saidpredetermined binary code; and means for cascading said electro-acousticsections in se ries, said means coupling current from an electrical padof one electro-acoustic section to an electrical pad of the succeedingelectro-acoustic section and controlling the phase of respectiveelectro-acoustic sections to correspond to said predetermined binarycode, the combination of successive electroacoustic sections effectingsaid predetermined binary code and effectively cascading the electricalimpedance of said electro-acoustic sections in series.

2. A relatively large input impedance interdigitated surface wavetransducer that is phase coded to correspond to a predetermined binarycode comprising in combination:

a. a piezoelectric substrate;

b. a plurality of spaced apart electro-acoustic sections disposed onsaid substrate to define a single acoustic channel, each of saidsections comprising two electrical pads having at least oneinterdigitated electrode pair selectively connected thereto andcorresponding to at least one binary bit of said predetermined binarycode, said at least one electrode pair having a relatively small valueof electrical impedance, at least one of said electroacoustic sectionshaving a total number of electrode pairs that is larger than at leastone other electro-acoustic section of said plurality of electro-acousticsections;

c. amplitude weighting means to control the effective impedance of eachof said electro-acoustic sections to be substantially the same; and

d. means for cascading said acoustic sections in series, said meansefiectively coupling current from an electrical pad of oneelectro-acoustic section to the succeeding electroacoustic sections andcontrolling the phase of respective electro-acoustic sections tocorrespond to respective binary numbers of said predetermined binarycode, whereby the effective impedance of each electro-acoustic sectionis the same and is cascaded in series with the impedance of adjacentelectro-acoustic sections to produce a phase coded interdigitatedsurface wave transducer having a relatively large input impedance.

3. An interdigital surface wave transducer having a relatively largeimpedance comprising in combination:

a. a piezoelectric substrate;

b. a plurality of spaced apart electro-acoustic sections formed on asurface of said substrate, each section being defined by a first arrayof electrodes commonly connected to a first electrical pad and a secondarray of electrodes commonly connected to a second electrical pad, saidfirst and second arrays of electrodes being interleaved to form aninterdigitated patterns that defines a preselected number of equal widthelectric field interaction regions in the surface of said substrate,each of said acoustic sections exhibiting a relatively small impedance,said plurality of electro-acoustic sections disposed in alignment todefine an acoustic channel; and

c. means for electrically coupling adjacent acoustic sections wherebythe electrical impedance of said plurality of electric sections add inseries to produce an interdigital surface wave transducer having arelatively large input impedance.

4. A phase coded transducer as set forth in claim 2 wherein said atleast one section having a larger number of electrode pairs has asmaller electrode interaction length than said at least one othersection.

5. A phase coded transducer as set forth in claim 2 wherein the acousticsection of said phase coded transducer having the minimum number ofelectrode pairs has the maximum electrode interaction length andacoustic sections having a larger number of electrode pairs have anelectrode interaction length that is a fraction of said maximum length,said fraction corresponding to the number of active electro-acousticinteractions per section.

6. An interdigital surface wave transducer in accordance with claim 3wherein said coupling means are arranged to interconnect successiveeIectro-acoustic sections acoustically in phase with each other.

7. An interdigital surface wave transducer in accordance with claim 6wherein said coupling means comprises an electrode three-fourths of anacoustic wavelength in width spaced between adjacent interaction regionsof successive electroacoustic sections, one end of said electrode beingconnected to an electrical pad common to both of said successiveacoustic sections.

8. An interdigital-surface wave transducer in accordance with claim 3wherein said coupling means are operative to reverse the phase ofsuccessive electro-acoustic sections.

9. An interdigitated surface wave transducer in accordance with claim 8wherein said coupling means comprises an electrode one-fourth of anacoustic wavelength in width spaced between adjacent interaction regionsof said successive electro-acoustic sections, one end of said electrodebeing connected to an electrical pad common to both of said successiveelectro-acoustic sections.

10. An interdigital surface wave transducer in accordance with claim 6wherein said coupling means comprises an interconnecting electrode, oneend of said electrode being electrically connected to an electrical padof one of said successive electro-acoustic sections and the other end ofsaid electrode being electrically connected to an electrical pad of theother of said successive elecu'o-acoustic sections, adjacent interactionregions of said successive electro-acoustic sections being spaced apartby three-fourths of an acoustic wavelength.

11. An interdigitated surface wave transducer as set forth in claim 10wherein said interconnecting electrode is one-fourth of an acousticwavelength in width.

12. An interdigitated surface wave transducer as set forth in claim 10wherein said interconnecting electrode is threefourths of an acousticwavelength in width.

13. An interdigitated surface wave transducer as set forth in claim 8wherein said coupling means comprises an interconnecting electrode, oneend of said electrode being electrically connected to an electrical padof one of said successive electro-acoustic sections and the other end ofsaid electrode being electrically connected to an electrical pad of theother of said successive electro-acoustic sections, adjacent interactionregions of said successive electro-acoustic sections being spaced apartby one-fourth of an acoustic wavelength.

14. A phase coded transducer system having a relatively large inputimpedance comprising in combination;

a. a piezoelectric substrate;

b. a first transducer including a plurality of spaced apartelectro-acoustic sections disposed on said substrate to define a singleacoustic channel, each of said sections comprising two electrical padshaving at least one interdigitated electrode pair selectively connectedthereto and corresponding to at least one binary bit of a predeterminedbinary code, said at least one electrical pair having a relatively smallvalue of electrical impedance, at least one of said electro-acousticsections having a total number of electrode pairs that is larger than atleast one other elecu'o-acoustic section of said plurality ofelectroacoustic sections, the acoustic section of said phase codedtransducer having the minimum number of electrode pairs having a maximumelectrode interaction length and acoustic sections having a largernumber of electrode pairs having an electrode interaction length that isa fraction of said maximum length, said fraction corresponding to thenumber of active electro-acoustic interactions per section;

c. means for cascading said acoustic section in series, said meanselectrically coupling one electro-acoustic section to the succeedingelectro-acoustic sections and controlling the phase of respectiveelectro-acoustic sections to correspond to respective binary members ofsaid predetermined binary code, whereby the efiective impedance of eachelectro-acoustic section is the same and is cascaded in series with theimpedance with the adjacent electro-acoustic section to produce a phasecoded interdigitated surface wave transducer having a relatively largeinput impedance; and

d. a second transducer on said substrate spaced from said firsttransducer and aligned with said acoustic channel, said secondtransducer having an electrode interaction length corresponding to saidminimum interaction length of said first transducer.

15. A phase coded transducer system as set forth in claim 14 whereinsaid means for cascading, selectively includes phase trically connectedto an electrical pad of one of successive electro-acoustic sections andthe other end of said electrode being electrically connected to anelectrical pad of the other of successive electro-acoustic sections,adjacent interaction regions of said successive electro-acousticsections being spaced apart by three-fourths of an acoustic wavelength.

II l l

1. A relatively large input impedance interdigital surface wavetransducer that is phase coded to correspond to a predetermined binarycode, comprising in combination: a. a piezoelectric substrate; b. aplurality of spaced apart electro-acoustic sections disposed on saidsubstrate to define a single acoustic channel, each of said sectionscomprising two electrical pads having at least one interdigitatedelectrode pair selectively connected thereto and having a relativelysmall electrical impedance, the relative phase of each of saidelectro-acoustic sections corresponding to a binary bit of saidpredetermined binary code; and c. means for cascading saidelectro-acoustic sections in series, said means coupling current from anelectrical pad of one electro-acoustic section to an electrical pad ofthe succeeding electro-acoustic section and controlling the phase ofrespective electro-acoustic sections to correspond to said predeterminedbinary code, the combination of successive electro-acoustic sectionseffecting said predetermined binary code and effectively cascading theelectrical impedance of said electro-acoustic sections in series.
 2. Arelatively large input impedance interdigitated surface wave transducerthat is phase coded to correspond to a predetermined binary codecomprising in combination: a. a piezoelectric substrate; b. a plUralityof spaced apart electro-acoustic sections disposed on said substrate todefine a single acoustic channel, each of said sections comprising twoelectrical pads having at least one interdigitated electrode pairselectively connected thereto and corresponding to at least one binarybit of said predetermined binary code, said at least one electrode pairhaving a relatively small value of electrical impedance, at least one ofsaid electro-acoustic sections having a total number of electrode pairsthat is larger than at least one other electro-acoustic section of saidplurality of electro-acoustic sections; c. amplitude weighting means tocontrol the effective impedance of each of said electro-acousticsections to be substantially the same; and d. means for cascading saidacoustic sections in series, said means effectively coupling currentfrom an electrical pad of one electro-acoustic section to the succeedingelectro-acoustic sections and controlling the phase of respectiveelectro-acoustic sections to correspond to respective binary numbers ofsaid predetermined binary code, whereby the effective impedance of eachelectro-acoustic section is the same and is cascaded in series with theimpedance of adjacent electro-acoustic sections to produce a phase codedinterdigitated surface wave transducer having a relatively large inputimpedance.
 3. An interdigital surface wave transducer having arelatively large impedance comprising in combination: a. a piezoelectricsubstrate; b. a plurality of spaced apart electro-acoustic sectionsformed on a surface of said substrate, each section being defined by afirst array of electrodes commonly connected to a first electrical padand a second array of electrodes commonly connected to a secondelectrical pad, said first and second arrays of electrodes beinginterleaved to form an interdigitated patterns that defines apreselected number of equal width electric field interaction regions inthe surface of said substrate, each of said acoustic sections exhibitinga relatively small impedance, said plurality of electro-acousticsections disposed in alignment to define an acoustic channel; and c.means for electrically coupling adjacent acoustic sections whereby theelectrical impedance of said plurality of electric sections add inseries to produce an interdigital surface wave transducer having arelatively large input impedance.
 4. A phase coded transducer as setforth in claim 2 wherein said at least one section having a largernumber of electrode pairs has a smaller electrode interaction lengththan said at least one other section.
 5. A phase coded transducer as setforth in claim 2 wherein the acoustic section of said phase codedtransducer having the minimum number of electrode pairs has the maximumelectrode interaction length and acoustic sections having a largernumber of electrode pairs have an electrode interaction length that is afraction of said maximum length, said fraction corresponding to thenumber of active electro-acoustic interactions per section.
 6. Aninterdigital surface wave transducer in accordance with claim 3 whereinsaid coupling means are arranged to interconnect successiveelectro-acoustic sections acoustically in phase with each other.
 7. Aninterdigital surface wave transducer in accordance with claim 6 whereinsaid coupling means comprises an electrode three-fourths of an acousticwavelength in width spaced between adjacent interaction regions ofsuccessive electro-acoustic sections, one end of said electrode beingconnected to an electrical pad common to both of said successiveacoustic sections.
 8. An interdigital surface wave transducer inaccordance with claim 3 wherein said coupling means are operative toreverse the phase of successive electro-acoustic sections.
 9. Aninterdigitated surface wave transducer in accordance with claim 8wherein said coupling means comprises an electrode one-fourth of anacoustic wavelength in width spaced between adjaceNt interaction regionsof said successive electro-acoustic sections, one end of said electrodebeing connected to an electrical pad common to both of said successiveelectro-acoustic sections.
 10. An interdigital surface wave transducerin accordance with claim 6 wherein said coupling means comprises aninterconnecting electrode, one end of said electrode being electricallyconnected to an electrical pad of one of said successiveelectro-acoustic sections and the other end of said electrode beingelectrically connected to an electrical pad of the other of saidsuccessive electro-acoustic sections, adjacent interaction regions ofsaid successive electro-acoustic sections being spaced apart bythree-fourths of an acoustic wavelength.
 11. An interdigitated surfacewave transducer as set forth in claim 10 wherein said interconnectingelectrode is one-fourth of an acoustic wavelength in width.
 12. Aninterdigitated surface wave transducer as set forth in claim 10 whereinsaid interconnecting electrode is three-fourths of an acousticwavelength in width.
 13. An interdigitated surface wave transducer asset forth in claim 8 wherein said coupling means comprises aninterconnecting electrode, one end of said electrode being electricallyconnected to an electrical pad of one of said successiveelectro-acoustic sections and the other end of said electrode beingelectrically connected to an electrical pad of the other of saidsuccessive electro-acoustic sections, adjacent interaction regions ofsaid successive electro-acoustic sections being spaced apart byone-fourth of an acoustic wavelength.
 14. A phase coded transducersystem having a relatively large input impedance comprising incombination; a. a piezoelectric substrate; b. a first transducerincluding a plurality of spaced apart electro-acoustic sections disposedon said substrate to define a single acoustic channel, each of saidsections comprising two electrical pads having at least oneinterdigitated electrode pair selectively connected thereto andcorresponding to at least one binary bit of a predetermined binary code,said at least one electrical pair having a relatively small value ofelectrical impedance, at least one of said electro-acoustic sectionshaving a total number of electrode pairs that is larger than at leastone other electro-acoustic section of said plurality of electro-acousticsections, the acoustic section of said phase coded transducer having theminimum number of electrode pairs having a maximum electrode interactionlength and acoustic sections having a larger number of electrode pairshaving an electrode interaction length that is a fraction of saidmaximum length, said fraction corresponding to the number of activeelectro-acoustic interactions per section; c. means for cascading saidacoustic section in series, said means electrically coupling oneelectro-acoustic section to the succeeding electro-acoustic sections andcontrolling the phase of respective electro-acoustic sections tocorrespond to respective binary members of said predetermined binarycode, whereby the effective impedance of each electro-acoustic sectionis the same and is cascaded in series with the impedance with theadjacent electro-acoustic section to produce a phase codedinterdigitated surface wave transducer having a relatively large inputimpedance; and d. a second transducer on said substrate spaced from saidfirst transducer and aligned with said acoustic channel, said secondtransducer having an electrode interaction length corresponding to saidminimum interaction length of said first transducer.
 15. A phase codedtransducer system as set forth in claim 14 wherein said means forcascading, selectively includes phase reversing means comprising anelectrode one-fourth an acoustic wave length in width spaced betweenadjacent interaction regions of successive electro-acoustic sections,one end of said electrode being connected to an electrical pad common toboth of said successive electro-acoustic sections.
 16. A phase codedtransducer system as set forth in claim 14 wherein said means forcascading, selectively includes an interconnecting electrode, one end ofsaid electrode being electrically connected to an electrical pad of oneof successive electro-acoustic sections and the other end of saidelectrode being electrically connected to an electrical pad of the otherof successive electro-acoustic sections, adjacent interaction regions ofsaid successive electro-acoustic sections being spaced apart bythree-fourths of an acoustic wavelength.